Chapter 17 Solutions

Chapter 17 - Alcohols and Phenols
Chapter Outline
I. Naming alcohols and phenols (Section 17.1).
1. Alcohols are classified as primary, secondary or tertiary, depending on the number
of organic groups bonded to the -OH carbon.
2. Rules for naming simple alcohols.
a, The longest chain containing the -OH group is the parent chain, and the parent
name replaces -e with -ol.
b. Numbering begins at the end of the chain nearer the -OH group.
c. The substituents are numbered according to their position on the chain and cited
in alphabetical order.
3. Phenols are named according to rules discussed in Section 15.1.
IL. Properties of alcohols and phenols (Section 17.2).
B. Hyogen bonding of alcohols and phenols.
Alcohols have ‘sp hybridization and a nearly tetrahedral bond angle.
Ay Alcohols and phenols have elevated boiling points, relative to hydrocarbons, due to
hydrogen bonding.
a. In hydrogen bonding, an -OH hydrogen is attracted to a lone pair of electrons
‘on another molecule, resulting in a weak electrostatic force that holds the
molecules together.
b. These weak forces must be overcome in boiling.
C Acidity and basicity of alcohols and phenols.
1. Alcohols and phenols are weakly acidic as well as weakly basic.
2. Alcohols and phenols dissociate to a slight extent to form alkoxide ions and
phenoxide ions.
3. Acidity of alcohols,
a. Alcohols are similar in acidity to water.
b. Alkyl substituents decrease acidity by preventing solvation of the alkoxide ion.
c. Electron-withdrawing substituents increase acidity by delocalizing negative
charge.
4. Alcohols don't react with weak bases, but they do react with alkali metals and
strong bases.
4. Acidity of phenols.
a, Phenols are a million times more acidic than alcohols and are soluble in dilute
NaOH.
. Phenol acidity is due to resonance stabilization of the phenoxide anion.
cc. Electron-withdrawing substituents increase phenol acidity, and electron-
donating substituents decrease phenol acidity.
IL. Alcohols (Sections 17.3 — 17.8).
A. Preparation of alcohols (Sections 17.3 — 17.5).
1. Familiar methods (Section 17.3).
a, Hydration of alkenes.
i. Hydroboration/oxidation yields non-Markoynikov products.
Oxymercuration/reduction yields Markovnikov products.
b. 1,2-diols can be prepared by OsO, hydroxylation, followed by reduction.
i. This reaction occurs with syn stereochemistry.
ii. Ring-opening of epoxides produces 1,2-diols with anti stereochemistry.
2. Reduction of carbonyl compounds (Section 17.4).
a. Aldehydes are reduced to primary alcohols.Alcohols and Phenols 403
b. Ketones are reduced to secondary alcohols.
Either NaBHg(milder) or LiAlH4(more reactive) can be used to reduce
aldehydes and ketones.
c. Carboxylic acids and esters are reduced to primary alcohols with LiAlHy.
i. These reactions occur by addition of hydride to the positively polarized
carbon of a carbonyl group.
ii, Water adds to the alkoxide intermediate during workup to yield alcohol
product,
3. Reaction of carbonyl compounds with Grignard reagents (Section 17.5).
a. RMgxX adds to carbonyl compounds to give alcohol products.
i, Reaction of RMgX with formaldehyde yields primary alcohols.
ii, Reaction of RMgX with aldehydes yields secondary alcohols,
iii, Reaction of RMgX with ketones yields tertiary alcohols,
iv. Reaction of RMgX with esters yields tertiary alcohols with at least two
identical R groups bonded to the alcohol carbon.
v. No reaction occurs with carboxylic acids because the acidic hydrogen
quenches the Grignard reagent.
. Limitations of the Grignard reaction,
i. Grignard reagents can't be prepared from reagents containing other reactive
functional groups.
ii, Grignard reagents cant be prepared from compounds having acidic
hydrogens.
c. Grignard reagents behave as carbon anions and add to the carbonyl carbon,
‘A proton from water is added to the alkoxide intermediate to produce the
alcohol.
B. Reactions of alcohols (Sections 17.6 - 17.8).
1. Conversion to alkyl halides (Section 17.6).
a, Tertiary alcohols (ROH) are converted to RX by treatment with HX.
The reaction occurs by an Syi mechanism.
. Primary alcohols are converted by the reagents PBr3 and SOCl.
The reaction occurs by an Sy2 mechanism.
2. Conversion into tosylates.
a. Reaction with p-toluenesulfonyl chloride converts alcohols to tosylates.
b. Only the O-H bond is broken.
¢. Tosylates behave as halides in substitution reactions.
d._ Sy2 reactions involving tosylates proceed with inversion of configuration.
3. Dehydration to yield alkenes.
a. Tertiary alcohols can undergo acid-catalyzed dehydration with warm aqueous
HpSO4.
i, Zaitsev products are usually formed.
ii, The severe conditions needed for dehydration of secondary and primary
alcohols restrict this method to tertiary alcohols.
iii, Tertiary alcohols react fastest because the intermediate carbocation formed in
this EI reaction is most stable.
b. Secondary and primary alcohols are dehydrated with POCI3 in pyridine.
i, This reaction occurs by an E2 mechanism.
ii, Pyridine serves as a base and as a solvent.
4, Conversion into esters.
5. Oxidation of alcohols (Section 17.7).
Primary alcohols can be oxidized to aldehydes or carboxylic acids.
Secondary alcohols can be oxidized to ketones.
Tertiary alcohols aren't oxidized.
Oxidation to ketones and carboxylic acids can be carried out with KMnOs,
CrO3, or NayCr307.
pose404° Chapter 17
e. Oxidation of a primary alcohol to an aldehyde is achieved with PCC.
PCC is also used on sensitive alcohols.
f. Oxidation occurs by a mechanism closely related to an E2 mechanism.
The reaction involves a chromate intermediate,
5. Protection of alcohols (Section 17.8).
a, It is sometimes necessary to protect an alcohol when it interferes with a reaction
involving a functional group in another part of a molecule.
The following reaction sequence may be applied:
i, Protect the alcohol,
ii, Carry out the reaction.
iii, Remove the protecting group.
¢. A trimethylsilyl (TMS) ether can be used for protection.
i, TMS ether formation occurs by an Sy2 route.
ii, TMS ethers are quite unreactive.
iii, TMS ethers can be cleaved by aqueous acid or by F-.
IIL, Phenols (Sections 17.9 - 17.10).
‘A. Preparation and uses of phenols (Section 17.9).
1. Phenols can be prepared by treating chlorobenzene with NaOH.
2. Phenols can also be prepared from isopropylbenzene (cumene).
a. Cumene reacts with O» by a radical mechanism to form cumene hydroperoxide.
b. Treatment of the hydroperoxide with acid gives phenol and acetone.
The mechanism involves protonation, rearrangement, loss of water,
readdition of water to form a hemiacetal, and breakdown to acetone and
phenol.
3. Chlorinated phenols, such as 2,4-D, are formed by chlorinating phenol.
4. BHT is prepared by Friedel-Crafts alkylation of p-cresol with 2-methylpropene.
B. Reactions of phenols (Section 17.10).
1. Phenols undergo electrophilic aromatic substitution reactions (Chapter 16).
‘The -OH group is a 0,p-director.
2. Strong oxidizing agents convert phenols to quinones.
a. Reaction with Fremy's salt to form a quinone occurs by a radical mechanism.
b. The redox reaction quinone > hydroquinone occurs readily.
c. Ubiquinones are an important class of biochemical oxidizing agents that
function as a quinone/hydroquinone redox system.
IV. Spectroscopy of alcohols an phenols (Section 17.11)
AIR spectroscopy.
1. Both alcohols and phenols show ~OH stretches in the region 3300-3600 cm”
a. Unassociated alcohols show a peak at 3600 cm™!.
b. Associated alcohols show a broader peak at 3300-3400 cm”.
2. Alcohols show a C-O stretch near 1050 cm”,
3. Phenols show aromatic bands at 1500-1600 cm™.
4. Phenol shows monosubstituted aromatic bands at 690 and 760 cm,
B. NMR ypectoscopy.
1 ‘C NMR spectroscopy, carbons bonded to ~OH groups absorb in the range
50-80 3
2. 'HINMR.
a, Hydrogens on carbons bearing -OH groups absorb in the range 3.5~4.5 6.
‘The hydroxy! hydrogen doesn't split these signals.
b. DO exchange can be used to locate the O-H signal.
c. Spin-spin splitting occurs between protons on the oxygen-bearing carbon and
neighboring -H.
Phenols show aromatic ring absorptions, as well as an O-H absorption in the
range 3-8 8.